Electric brake system

ABSTRACT

The present invention provides an electric brake system which detects the occurrence of overshooting, and corrects a feedback gain in the PID feedback control at overshooting. For example, it increases a differential gain Kd. Accordingly, a damping force increases from the time when the overshooting is determined, and after the overshooting, the output of an electric brake moderately decays to get closer to the target value with the passage of time.

CROSS REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of Japanese PatentApplication No. 2003-299894 filed on Aug. 25, 2003, the content of whichare incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to an electric brake system for electricallycontrolling brake pressure corresponding to the amount of operation ofthe brake operating member.

BACKGROUND OF THE INVENTION

Among conventional anti-skid control (ABS) apparatuses thatincrease/decrease the fluid-chamber capacity by means of linear movementcaused by an electric motor, one proposed apparatus is a type in whichan instruction current supplied to the motor is determined in accordancewith a state variable corresponding to a motor speed as well as a tireslip rate (see Japanese Patent Application Laid-open No. HEI6-72312).This apparatus determines the instruction current supplied to the motoranticipating the speed energy provided by the motor in order to suppressthe output from the actuator from overshooting a target value.

Further, in a linear electric disc brake which is operated by means oflinear movement caused by an electric motor, an upper limit and a lowerlimit of the rotational speed of the motor are predetermined and theactual rotational speed of the motor is suppressed within thepredetermined range (see Japanese Patent Application Laid-open No.2003-104195).

However, in the apparatus disclosed in Japanese Patent ApplicationLaid-open No. HEI6-72312, the higher the rotational speed of the motoris, that is, the faster the response is, the larger the resistance force(damping force) against the motor rotation becomes. Hence, the best useof the potential of the motor is not made. Accordingly, the following-upof the target value of the actuator output by the actual controlvariable delays, leading to a poor response.

In the brake apparatus disclosed in Japanese Patent ApplicationLaid-open No. 2003-104195, suppression on the rotational speed of themotor makes it impossible to adequately use the potential of the motor.Accordingly, the following-up of the target value of the actuator outputby the actual control variable delays, leading to a poor response.

Therefore, any apparatus used for brake control such as the twoapparatuses disclosed in the related art is incapable of providing theresponse required for the brake control.

On the other hand, if the adjustment of the instruction current inaccordance with the rotational speed of the motor or the setting of anupper limit and lower limit of the rotational speed of the motor is notexecuted, as in the cases of both apparatuses disclosed above in therelated art, this may make it possible to provide a responsecorresponding to the potential of the motor. However, it is impossibleto suppress an overshoot in which the output from the actuator exceedsor surpasses the target value, and/or hunting in which the output fromthe actuator attempts to converge on the target value resulting inoscillation phenomenon. For this reason, a phenomenon where a largebrake force is generated once and then rapidly drops occurs. Thisphenomenon causes a driver to feel uncomfortable with braking, whereby astable riding comfort is not provided.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide anelectric brake system which is capable of making use of the potential ofa motor to improve response, and also suppressing overshooting andhunting so as to offer drivers stable riding comfort.

An electric brake system using an electric motor to operate an electricbrake according to an aspect of the present invention, a target valuesetting portion sets a target value for an output from the electricbrake for generating a target brake force corresponding to the amount ofoperation of a brake operating member. Further, a feedback controlportion executes a feedback control to provide feedback on an actualcontrol variable for the electric brake and to reduce a differencebetween the target value and the actual control variable. When theelectric brake is operated so as to produce the target output value, ifan overshoot detection portion detects overshooting, a feedback controlvariable correction portion corrects a feedback control variable in thefeedback control portion.

For example, the feedback control portion may execute the feedbackcontrol based on PID. Further, the feedback control variable correctionportion corrects a feedback gain in the feedback control portion, andincreases a differential gain when the overshooting is detected, ascompared with before the overshooting is detected.

Alternatively, the feedback control portion executes the feedbackcontrol based on PID, and the feedback control variable correctionportion corrects the feedback gain in the feedback control portion. Whenthe overshooting is detected, the feedback control variable correctionportion remains a differential gain unchanged, and decrease a proportiongain and an integral gain as compared with before the overshooting isdetected.

In this manner, at the time of overshooting, the feedback controlvariable correction portion corrects the feedback control variable inthe feedback control. For example, it corrects a feedback gain in thePID feedback control. Accordingly, a damping force increases from thetime of determining that the overshooting occurs, and after theovershooting, the output from the electric brake moderately decays andgets closer to the target value.

Accordingly, it is possible to allow the actual control variable for theelectric brake to follow the target value with a good response.Furthermore, it is possible to suppress the oscillation phenomenon whichgenerally subsequently occurs due to inertia and elasticity of the motoror the electric brake after the following-up of the target value, andtherefore to maintain the stable output state. This in turn makes itpossible to make use of the potential of the motor, improve response,and suppress overshooting and hunting. Further, because it is possibleto eliminate, for example, feeling of acceleration/deceleration contraryto the driver's intention, the electric brake system is capable ofoffering stable riding comfort to the driver and also avoiding thefunctional depression, functional abnormality in the vehicle stabilitycontrol.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention will beunderstood more fully from the following detailed description made withreference to the accompanying drawings. In the drawings:

FIG. 1 is a schematic diagram illustrating an outline of a structure ofan electric brake system according to a first embodiment of the presentinvention;

FIG. 2 is a flow chart of instruction current setting processing whichis executed by the electric brake system shown in FIG. 1;

FIG. 3 is a flow chart showing in detail feedback gain correctionprocessing shown in FIG. 2;

FIG. 4 is a block diagram for a feedback control based on PID; and

FIG. 5 is a timing chart illustrating an operation of the electric brakesystem shown in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described further with reference tovarious embodiments in the drawings.

First Embodiment

FIG. 1 illustrates an outline of a structure of an electric brake systemto which an embodiment according to the present invention is applied.Hereafter, the structure of the electric brake system will be describedwith reference to FIG. 1.

The electric brake system includes a brake pedal 1 corresponding to abrake operating member, an electronic control unit (hereinafter referredto as “ECU”) 2, an electric brake 3 for generating a brake force on eachwheel, various sensors 50 a to 50 c, and the like. The electric brakes 3are mounted on the respective four wheels, but FIG. 1 representativelyillustrates only one of the electric brakes 3 mounted on the fourwheels.

To the brake pedal, 1 a pedal operating sensor 50 a is attached fordetecting an operation amount of the brake pedal 1, e.g., the pedaldepression force or the amount of stroke. A detection signal is sentfrom the pedal operating sensor 50 a to the ECU 2.

The ECU 2 is equipped with a computer 2 a, a drive circuit 2 b and thelike. The ECU 2 receives detection signals from the pedal operatingsensor 50 a, the wheel speed sensor 50 b provided for each wheel, andthe yaw-rate sensor 50 c, and uses the received detection signals toexecute various types of operations. To be more specific, the ECU 2 usesthe computer 2 a to obtain a target brake force to be generated by theelectric brake 3, that is, to obtain a target value for an instructioncurrent in accordance with a target deceleration to be generated for thevehicle, and outputs the instruction current equivalent to the obtainedtarget value from the drive circuit 2 b.

The electric brake 3 is a disc brake and has a disc rotor 4 rotating inconjunction with the wheel, a pair of friction pads 5 and 6, that is,the first and second brake pads, which are placed on both sides of thedisc rotor 4 so as to sandwich it, and a caliper 7 which straddles thedisc rotor 4 and holds the pair of friction pads 5 and 6. The caliper 7is structured as follows.

A motor 10 having a rotating shaft 10 a is secured inside a housing 8forming the outside shape of the caliper 7. The housing 8 incorporatescomponents for pressing the first and second brake pads 5 and 6 onto thedisc rotor 4.

An inner rotor 11 with an outer-tooth portion is secured on the rotatingshaft 10 a of the motor 10. An outer rotor 12 is placed around the innerrotor 11 and has an inner-tooth portion meshed with the outer-toothportion of the inner rotor 11. Roller bearings 13 and 14 are placed onboth end faces of the outer rotor 12. A clutch release member 15 isplaced so as to be adjacent to the roller bearing 14, and has a holeformed in a central portion thereof. A bearing 16 is provided between aninner wall face of the hole and an outer circumferential face of therotating shaft 10 a of the motor 10. Note that, hereafter, the directionof the axis center of the rotating shaft 10 a of the motor 10 isreferred to simply as “the axial direction”, and the direction ofrotation of the rotating shaft 10 is referred to as “the circumferentialdirection”. Further the direction of movement from the motor 10 towardthe disc rotor 4 is referred to as “the forward direction” and thereverse direction is referred to as “the backward direction”.

On the disc rotor 4 side of a distal end portion 10 b of the rotatingshaft 10 a of the motor 10, a female screw member 17 is placed andprovided with a clutch member 17 a which is in contact with the end faceof the distal end portion lob of the rotating shaft 10 a. A female-screwhole 17 b is formed in a central portion of the female screw member 17and a male screw 18 a is screwed thereinto. The male screw 18 a isformed as a part of a male screw member 18 provided for securing afriction-material holding portion 9 holding the first brake pad 5.

The rotating shaft 10 a of the motor 10 rotates around an axis centera1. An eccentric rotation portion 10 c is formed on the rotating shaft10 a and has an axis center a2 positioned away from the axis center a1by the amount of eccentricity E.

The eccentric rotation portion 10 c provided on the motor rotating shaft10 a; the inner rotor 11 rotated around the axis center a1 by therotation of the eccentric rotation portion 10 c; a pin hole 8 b formedin a motor accommodating wall 8 a of the housing 8; and the outer rotor12 form a cyclo-speed-reducer mechanism.

In the cyclo-speed-reducer mechanism, the eccentric rotation portion 10c serving as an input shaft is rotated by applying electricity to themotor 10. At this time, inner pins 11 a provided on the inner rotor 11are restrained from moving by the pin hole 8 b so as to be capable ofmoving only within the pin hole 8 b. Therefore, by the rotation of theeccentric rotation portion 10 c, an axis center a3 of each inner pin 11a revolves inside the pin hole 8 b and the inner rotor 11 revolves at acertain RPM around the axis center a2. The RPMs of the revolution of theinner pin 11 a and the inner rotor 11 at this point are equivalent tothe rotational speed of the motor 10.

The revolution of the inner rotor 11 is transmitted to the outer rotor12. Then the outer rotor 12 rotates in the circumferential direction ata gear ratio which is determined on the basis of the number of teeth ofthe inner-tooth portion of the outer rotor 12 and the number of teeth ofthe outer-tooth portion of the inner rotor 11. At this point, becausethe outer rotor 12 is sandwiched between the roller bearings 13 and 14,the rotating position of the outer rotor 12 in the axial direction isfixed.

Further, ramp portions 19, the roller bearing 14 and the clutch releasemember 15 form a ramp mechanism.

A plurality of ramp portions 19 are provided on the end face of theouter rotor 12 on the side of the bearing 14, and formed along thecircumferential direction of the outer rotor 12 so as to be inclined ata predetermined angle with respect to the end face of the outer rotor12, which is not shown in FIG. 1. Hence, when the ramp portion 19rotates with the rotation of the outer rotor 12, the roller bearing 14and the clutch release member 15 are urged toward the left side of thedrawing of FIG. 1 by this inclination of the ramp portion 19.

The clutch release member 15 is formed of a ring-shaped member the outercircumference thereof is in contact with a bearing 20 and the innercircumference thereof is in contact via the bearing 16 with the endportion 10 b of the motor rotating shaft 10 a. Hence, in thecircumferential direction and axial direction, the clutch release member15 can slide inside the housing 8. However, the clutch release member 15is positioned in contact with the roller bearing 14 on the face which isperpendicular to the axial direction, so as to be inhibited from movingtoward the roller bearing 14. Further, the clutch release member 15 isprovided with a circle shaped projection 15 a on the face thereofopposite to the outer rotor 12. The projection 15 a is placed in contactwith the clutch member 17 a provided on the female screw member 17 inthe axial direction.

In the ramp mechanism structured in this manner, the rotation of theouter rotor 12 is transmitted via the roller bearing 14 to the clutchrelease member 15. Then, while the clutch release member 15 is out ofcontact with the female screw member 17 (clutch member 17 a), the clutchrelease member 15 rotates integrally with the roller bearing 14 and theouter rotor 12 at the same speed in the circumferential direction.

Then, the projection 15 a of the clutch release member 15 comes intocontact with the female screw member 17 (clutch member 17 a) and therestraining force acts on the clutch release member 15. Thereupon, aspeed differential in rotational speed in the circumferential directionis produced between the clutch release member 15 and the outer rotor 12.Hence, the relative displacement between the clutch release member 15and the outer rotor 12 in the axial direction is increased in accordancewith the speed differential and the inclined angle of the ramp portion19. Thus the clutch release member 15 moves towards the left in FIG. 1.

The female screw member 17 is able to rotate relatively with respect tothe housing 8, and is provided with a plurality of clutch members 17 acapable of coming in and out of contact with the shaft end portion 10 bof the motor rotating shaft 10 a.

The clutch members 17 a are arranged in plurality in the circumferentialdirection, and each extend in the direction of the center of the femalescrew member 17. When the end portion of the clutch member 17 a is incontact with the shaft end portion 10 b at a contact portion 10 d, therotation of the motor rotating shaft 10 a is transmitted to the femalescrew member 17 because of a friction force generated on the contactportion 10 d, and thus the female screw member 17 rotates in thecircumferential direction. Accordingly, because the male screw member 18is inhibited from rotating in the circumferential direction, the malescrew member 18 moves in the axial direction by the rotation of thefemale screw member 17, and presses the first brake pad 5 onto the discrotor 4.

Note that, a one-way clutch 21 permitting the female screw member 17 torotate only in one direction is provided on the outer circumferentialportion of the female member 17, so as to engage with the housing 8. Theone-way clutch 21 permits the rotation of the female screw member 17 inthe direction that thrusts the first brake pad 5 and blocks the rotationthereof in the reverse direction. For this reason, when releasing thebrake, the direction of rotation of the female screw member 17 is notreversed. Accordingly, even if wearing of the first brake pad 5develops, it is possible to prevent the space between the first brakepad 5 and the disc rotor 4 from exceeding a predetermined value.

Further, the male screw member 18 is inhibited from moving in thecircumferential direction and is capable of moving only in the axialdirection. One end of the male screw member 18 is joined to thefriction-material holding portion 9 holding the first brake pad 5, andat the other end thereof a male screw 18 a is formed. The male screw 18a is located to be screwed into the female screw 17 b of the femalescrew member 17, so that the axis center thereof is aligned with theaxis center al of the motor rotating shaft 10 a.

With the electric brake 3 structured as described hitherto, when theinstruction current determined by the ECU 2 is output from the drivecircuit 2 b, the motor 10 receives the instruction current and isdriven. Accordingly, as the motor rotating shaft 10 a is rotated, themale screw member 18 is urged toward the left side of the drawing ofFIG. 1, to cause the first brake pad 5 to come into contact with thedisc rotor 4. Then, upon contact of the first brake pad 5 with the discrotor 4, the male screw member 18 is incapable of moving any furthertoward the left side of the drawing, so that the entire caliper 7 isurged toward the right side of the drawing of FIG. 1 by the urging forceof the male screw member 18 toward the left side of the drawing to bringthe second brake pad 6 into contact with the disc rotor 4. In thismanner, the disc rotor 4 is clamped between the first and second brakepads 5 and 6, and a friction force is applied to the disc rotor 4 so asto stop the rotation. Thus, the rotation of the wheel in conjunctionwith the disc rotor 4 is stopped, and a brake force is obtained by afriction force generated between the tire mounted on the wheel and theroad surface.

The foregoing electric brake system executes the processing of settingan instruction current to be applied to the motor 10 of the electricbrake 3. FIG. 2 shows a flow chart of the processing of setting theinstruction current executed by the ECU 2. The instruction currentsetting processing is described with reference to FIG. 2.

The instruction current setting processing shown in the flow chart inFIG. 2 is executed whenever the ignition switch (not shown) is turnedon. First, at 100 in FIG. 2, the determination whether a pedal input isproduced or not is made. This determination of the processing is made onthe basis of a signal from the pedal operating sensor 50 a which isindicative of whether or not the brake pedal 1 is operated.

Then, if the determination that a pedal input is produced is made, theprocedure proceeds to processing at 110 to calculate, on the basis ofthe signal from the pedal operating sensor 50 a, a target decelerationin accordance with the pedal input, namely, the pedal depression force,pedal speed or the like. Then the procedure proceeds to processing at120. On the other hand, if there is no pedal input, it means there is nobrake request from the driver. Therefore, the procedure proceedsdirectly to processing at 120.

At 120, the determination is made whether or not a slip rate calculatedon the basis of a detection signal from the wheel speed sensor 50 b anda yaw rate calculated on the basis of a detection signal from the yawrate sensor 50 c fall within specified value ranges. The “specifiedvalue” referred to herein means the degree of slip rate or yaw rateallowing the ABS control, vehicle stability control and the like to beexecuted.

Then, if an affirmative determination is made at 120, it is assumed thatthe ABS control, vehicle stability control and the like are notexecuted, and the procedure proceeds to processing at 130. On the otherhand, if a negative determination is made, it is assumed that the ABScontrol, vehicle stability control and the like are executed, and theprocedure proceeds to processing at 140.

Note that, regarding the slip rate and the yaw rate, the operation forobtaining the slip rate and/or the yaw rate is generally executed inseparated processing for executing ABS control and the like. Therefore,in the first embodiment, those operation results are used.

A target brake force is calculated for each wheel at 130. Morespecifically, the brake force for each wheel required for achieving thetarget deceleration which has been obtained at 120 is calculated.Further, a target value for an output from the electric brake 3 inaccordance with the target brake force is set. The target value servesas a target control variable which is required to be output to theelectric brake 3 by the ECU 2. On the other hand, at 140, in order toexecute the ABS control, vehicle stability control and/or the like, atarget brake force for each wheel in accordance with each of the abovecontrols is calculated, and a target value for an output from theelectric brake 3 in accordance with the calculated target brake force iscalculated.

Then, at 150, the ECU 2 executes feedback gain correction processing ascorrection for a feedback control variable. The feedback gain correctionprocessing is executed for correction of the target value of the outputfrom the electric brake 3, with the objective of preventing theoccurrence of overshooting or hunting in the instruction current. FIG. 3shows a flow chart representing the feedback gain correction processingin detail. The following description is given of the feedback gaincorrection processing with reference to FIG. 3.

In the feedback gain correction processing, first, at 200 in FIG. 3, thedetermination is made whether or not the amount of variation in targetvalue falls within a specified value range (first specified valuerange). For example, the determination is made whether or not the amountof the changes when the target value for the output from the electricbrake 3 is converted to a deceleration, that is, the amount of thechanges in deceleration which will be produced when the electric brake 3produces an output corresponding to the target value, falls within ±0.1G/sec. Thus, it is possible to determine whether or not the target valuefalls within a stable state. The state in which the target value isstable as defined herein means a state in which the target valueincreases at a predetermined gradient to reach the required value andthen is retained at a substantially constant value.

Accordingly, when the amount of variation in the target value is largeand the target value falls out of the stable state, the state is not ina stage where overshooting or hunting occurs. Thus, without anyprocessing, the feedback gain correction processing is terminated. Onthe other hand, when the amount of variation in the target value issmall and the target value still falls within the stable state, thedetermination is made that the state is in a stage where overshootingoccurs and will be shifted to hunting, and the procedure proceeds toprocessing at 210.

At 210, the absolute value (|target brake force−control variable|) ofthe difference between the target value for the output from the electricbrake 3 and the control variable (output) actually generated isobtained, and then the determination is made whether or not the absolutevalue of the difference falls within another specified value range(second specified value range). For example, the determination is madewhether, when the absolute value of the difference is converted into adeceleration, the deceleration is within a 0.1 G or not. That is, evenif the target brake force is in the stable state, when the controlvariable actually generated differs greatly from the target value,feedback gain correction processing must not be executed in order tomake the actual control variable approximate to the target brake force.Accordingly, if an affirmative determination is made at 210, theprocedure proceeds to processing at 220. If a negative determination ismade, without any processing, the feedback gain correction processing isterminated.

At 220, the determination is made whether or not increase/decreasetrends in the target value for the output from the electric brake 3 andthose in the control variable actually generated are in agreement. Inthis connection, the determination is made, regarding the“increase/decrease trends” referred to herein, it is assumed that whenthe target value and/or the actual control variable is in the stablestate after an increase or after a decrease, the trend of the targetvalue and/or the actual control variable before entering the stablestate is continued without change.

When both the target value and the actual control variable are on theincrease trend or on the decrease trend, this means the state before anyovershooting or hunting occurs, or alternatively the state in which theincrease/decrease trends are in agreement with each other again afterovershooting or hunting has occurred. Further, when the target value forthe output from the electric brake 3 and the actual control variable arenot in agreement with each other in increase/decrease trends, this meansthe state in which the brake system goes into oscillation mode caused byhunting after overshooting has occurred.

Hence, if a negative determination is made at 220, it is assumed thatafter overshooting has occurred, and the brake system goes into theoscillation mode caused by hunting, the ECU 2 sets an overshootdetermination flag and the procedure proceeds to processing at 230.Then, the feedback gain correction is executed. More specifically, whenovershooting occurs, correction is made for a feedback gain in thefeedback control based on PID executed at 160 in FIG. 2, which will bedescribed later.

Here, the PID feedback control is described. The term “PID” is anacronym for “Proportion”, “Integral” and “Differential”, and refers toexecution of the proportion, integral and differential controls in sucha way that feedback is executed on the actual control variable withrespect to the target value and the difference between the actualcontrol variable and target value becomes zero. The PID feedback controlcan be illustrated as in FIG. 4.

As shown in FIG. 4, the above controls are individually executed on thedifference between the target value and the actual control variable todetermine feedback gains (proportion gain Kp, integral gain Ki anddifferential gain Kd).

In the proportion control, a control input is executed proportional tothe difference (error) between the target value and the actual controlvariable. When executing such a control, the control gain may possiblybe raised in order to increase the response speed, but this makes theactual control variable oscillational. Therefore, even in the zero stateof the difference between the target value and the actual controlvariable, the integral control makes it possible to maintain the stateof the difference. The integral term increases with time.

The execution of these proportional and integral controls makes itpossible to come close to the target. However, it takes a long time tocome close to the target and the response speed is slowed down. Hence,the differential control is executed to increase the control input whena large difference between the target value and the actual controlvariable is produced, and to decrease the control input when a smalldifference is produced. Accordingly, it becomes possible to bring theactual control variable closer to the target for stability.

Because of this, the first embodiment employs a PID feedback control tomake the actual control variable approximate to the target value, inwhich a proportion gain Kp, an integral gain Ki and a differential gainKd are preset as values determined in view of response and the like.

Accordingly, at 160 in FIG. 2, the correction, for example, increasingthe differential gain Kd of the feedback gains set as described aboveand making no change to the proportion gain Kp and the integral gain Kiis executed.

On the other hand, if an affirmative determination is made at 220, theprocedure proceeds to processing at 240 to determine whether a presentstate is the state before any overshooting or hunting occurs or thestate in which hunting introduces oscillation mode after overshootinghas occurred once, and then the increase/decrease trends are inagreement with each other again. The determination in this processing isdependent on whether or not the overshoot determination flag having beenset at 220 is set.

Even if the target value for the output from the electric brake 3 andthe actual control variable are once in agreement with each other inincrease/decrease trends after overshooting has occurred and thenhunting has introduced oscillation mode, there is a necessity tosubsequently continue executing the feedback gain correction. Hence, ifthe overshoot determination flag is set, the determination is made thatthere is a necessity to continue still executing the feedback gaincorrection, and the procedure proceeds to processing at 230 to executethe feedback gain correction as in the above case.

Note that, if a negative determination is made at 200 or at 210, thedetermination is made that there is no necessity any further to executethe feedback gain correction, or alternatively that circumstancesrequire executing first of all the setting of the target brake force inaccordance with the pedal input. Therefore, the overshoot determinationflag is reset.

In this manner, the feedback gain correction processing is terminated.After that, the procedure proceeds to processing at 160 in FIG. 2 toobtain a difference between the target value for the output from theelectric brake 3 and the actual control variable. Then, the PID feedbackcontrol in which the target value is adjusted so as to decrease theobtained difference is executed.

Then, by converting the target value for the output from the electricbrake 3 into the corresponding instruction current, the setting of theinstruction current is completed. Then, the set instruction current isoutput from the ECU 2 to the motor 10, and thus the electric brake 3produces the output to be equivalent to the set target value.

FIG. 5 shows changes in the target value for the output from theelectric brake 3 and the actual control variable in the electric brakesystem in which the foregoing processing is executed. Note that thedotted line in FIG. 5 indicates conventional changes in a target valueand an actual control variable when the aforementioned feedback gaincorrection processing is not executed, for reference purposes.

As shown in FIG. 5, the target value for the output from the electricbrake 3 increases up to a value corresponding to the required targetbrake force with a predetermined increasing gradient. With followingthis target value, the actual control variable increases rapidly.However, due to the influence of inertia and elasticity of the motor 10and the like, the actual control variable increases up to a valuecorresponding to the required target brake force as in the case of thetarget value, and then overshoots this value.

For this reason, if the target control variable does not undergo thefeedback gain correction processing as in the case of the conventionalexample, after inertia energy (speed energy) of the motor or theelectric brake has completely replaced elastic energy, due to the storedelastic energy the actual control variable commences decreasing againtoward the target value. This phenomenon is repeated to result in adamped oscillation phenomenon of the target control variable repeatedlymoving up and down with respect to the target value and finallystabilizing at the target value.

However, in the first embodiment, if the overshooting is detected, thefeedback gain is corrected. More specifically, the differential gain Kdis increased. Therefore, as shown in FIG. 5, the damping force startsincreasing at the time when the overshooting is detected, and after theactual control variable has overshoot, the output from the electricbrake 3 moderately decays to get closer to the target value with thepassage of time with avoidance of the oscillation phenomenon as occursin the conventional example.

Accordingly, it is possible to allow the actual control variable for theelectric brake 3 to follow the target value with a good response.Furthermore, it is possible to suppress the oscillation phenomenon whichis generally caused by inertia and elasticity of the motor 10 or theelectric brake 3 after the following-up of the target value, andtherefore to maintain the stable output state. This in turn makes itpossible to make use of the potential of the motor, improve response,and suppress overshooting and hunting. Further, because it is possibleto eliminate, for example, feeling of acceleration/deceleration contraryto the driver's intention, the electric brake system is capable ofoffering stable riding comfort to the driver and also avoiding thefunctional depression, functional abnormality in the vehicle stabilitycontrol.

Note that because after the overshooting, the output from the electricbrake 3 decays merely moderately from the value showing whenovershooting, the target brake force required temporary and the actualbrake force are not in agreement with each other. However, for example,a brake force larger than the target value is generated, and then thebrake force is retained. Therefore, even when the brake force and thetarget value are not in agreement, the driver is not caused to feel achange in brake force which is contrary to the driver's intention. Inother words, when hunting occurs, the brake force varies withoscillation twice the magnitude of the difference between the targetvalue for the output from the electric brake 3 and an overshoot. Hence,a change in which, the brake force is sharply reduced after a largebrake force is generated, is introduced and causes the driver to feeluncomfortable. However, in the first embodiment, a brake force slightlydiffering from the target brake force is retained, whereby the driverdoes not feel uncomfortable.

Further, if a permissible range is set for a difference between therequired target brake force and the actual brake force, it is possibleto further mitigate the driver's uncomfortable feeling.

OTHER EMBODIMENTS

In the first embodiment, even when the vehicle behavior control such asthe ABS control and the like is executed, the target brake force foreach wheel is calculated and the target value for the output from theelectric brake 3 is calculated (at 140 in FIG. 2), and then the feedbackgain correction processing is carried out (at 150 in FIG. 2). However,when the vehicle behavior control such as the ABS control and the likeis executed actually, it is possible to give more priority to thevehicle behavior control such as the ABS control and the like andsuspend execution of the feedback gain correction processing. In thiscase, for example, at the time when the vehicle behavior control isexecuted, a flag showing this execution may be set, and then thedetermination whether or not the vehicle behavior control is beingexecuted may be made prior to the determination at 200 in FIG. 3. If thevehicle behavior control is executed, the feedback gain correctionprocessing may be terminated without any processing.

Further, in the above second embodiment, the differential gain Kd isincreased in the feedback gain correction processing. However, it isalso possible to execute another correction processing in which thedifferential gain Kd is not changed and the proportion gain Kp and theintegral gain Ki are decreased.

Note that each of the procedures of the flow charts describing theaforementioned procedure corresponds to a portion for implementing theprocessing executed in procedure.

While the above description is of the preferred embodiments of thepresent invention, it should be appreciated that the invention may bemodified, altered, or varied without deviating from the scope and fairmeaning of the following claims.

1. An electric brake system using an electric motor to operate anelectric brake, comprising: a target brake force setting portion forsetting a target brake force corresponding to the amount of operation ofa brake operating member; a target value setting portion for setting atarget value for an output of said electric brake for generating thetarget brake force set by said target brake force setting portion; afeedback control portion for executing a feedback control for providingfeedback on an actual control variable for said electric brake, andminimizing a difference between said target value and the actual controlvariable; overshoot detection portions for operating said electric braketo produce the output of said target value from said electric brake, anddetecting occurrence of overshooting when the actual control variablefor the electric brake overshoots said target value; and a feedbackcontrol variable correction portion for correcting a feedback controlvariable in said feedback control portion when said overshoot detectionportion detects the overshooting.
 2. The electric brake system accordingto claim 1, wherein said overshoot detection portion has a portion fordetermining whether or not the amount of variation in said target valuefalls within a first specified value range, and when the determinationthat the amount of variation in said target value falls outside saidfirst specified value range is made, it is determined that overshootinghas not occurred.
 3. The electric brake system according to claim 2,wherein said overshoot detection portion has a portion for determiningwhether or not the difference between said target value and said actualcontrol variable falls within a second specified value range, and whenthe determination that the difference between said target value and saidactual control variable falls outside said second specified value range,it is determined that overshooting has not occurred.
 4. The electricbrake system according to claim 3, wherein said overshoot detectionportion has a portion for determining whether or not anincrease/decrease trend in said target value and that in said actualcontrol variable are in agreement with each other, and determines thatsaid overshooting occurs when the increase/decrease trend in said targetvalue and that in said actual control variable are not in agreement witheach other.
 5. The electric brake system according to claim 2, whereinsaid overshoot detection portion has a portion for determining whetheror not an increase/decrease trend in said target value and that in saidactual control variable are in agreement with each other, and determinesthat said overshooting occurs when the increase/decrease trend in saidtarget value and that in said actual control variable are not inagreement with each other.
 6. The electric brake system according toclaim 2, further comprising a determination portion for determining thatsaid overshooting has occurred if said overshoot detection portion hasalready detected said overshooting when said target value and saidactual control variable are in agreement in an increase/decrease trend,wherein when said determination portion determines that saidovershooting has been detected, said feedback control variablecorrection portion executes correction on said feedback controlvariable.
 7. The electric brake system according to claim 2, whereinsaid feedback control portion executes the feedback control based onPID, and said feedback control variable correction portion corrects thefeedback gain in said feedback control portion, and when saidovershooting is detected, a differential gain is increased as comparedwith the differential gain shown before said overshooting is detected.8. The electric brake system according to claim 2, wherein said feedbackcontrol portion executes the feedback control based on PID, and saidfeedback control variable correction portion corrects the feedback gainin said feedback control portion, and when said overshooting isdetected, a differential gain is not changed and at least one of aproportion gain and an integral gain is decreased as compared with thegains shown before said overshooting is detected.
 9. The electric brakesystem according to claim 1, wherein said overshoot detection portionhas a portion for determining whether or not the difference between saidtarget value and said actual control variable falls within a secondspecified value range, and when the determination that the differencebetween said target value and said actual control variable falls outsidesaid second specified value range, it is determined that overshootinghas not occurred.
 10. The electric brake system according to claim 9,wherein said overshoot detection portion has a portion for determiningwhether or not an increase/decrease trend in said target value and thatin said actual control variable are in agreement with each other, anddetermines that said overshooting occurs when the increase/decreasetrend in said target value and that in said actual control variable arenot in agreement with each other.
 11. The electric brake systemaccording to claim 9, further comprising a determination portion fordetermining that said overshooting has occurred if said overshootdetection portion has already detected said overshooting when saidtarget value and said actual control variable are in agreement in anincrease/decrease trend, wherein when said determination portiondetermines that said overshooting has been detected, said feedbackcontrol variable correction portion executes correction on said feedbackcontrol variable.
 12. The electric brake system according to claim 9,wherein said feedback control portion executes the feedback controlbased on PID, and said feedback control variable correction portioncorrects the feedback gain in said feedback control portion, and whensaid overshooting is detected, a differential gain is increased ascompared with the differential gain shown before said overshooting isdetected.
 13. The electric brake system according to claim 9, whereinsaid feedback control portion executes the feedback control based onPID, and said feedback control variable correction portion corrects thefeedback gain in said feedback control portion, and when saidovershooting is detected, a differential gain is not changed and atleast one of a proportion gain and an integral gain is decreased ascompared with the gains shown before said overshooting is detected. 14.The electric brake system according to claim 1, wherein said overshootdetection portion has a portion for determining whether or not anincrease/decrease trend in said target value and that in said actualcontrol variable are in agreement with each other, and determines thatsaid overshooting occurs when the increase/decrease trend in said targetvalue. and that in said actual control variable are not in agreementwith each other.
 15. The electric brake system according to claim 14,further comprising a determination portion for determining that saidovershooting has occurred if said overshoot detection portion hasalready detected said overshooting when said target value and saidactual control variable are in agreement in the increase/decreasetrends, wherein when said determination portion determines that saidovershooting has been detected, said feedback control variablecorrection portion executes correction on said feedback controlvariable.
 16. The electric brake system according to claim 14, whereinsaid feedback control portion executes the feedback control based onPID, and said feedback control variable correction portion corrects thefeedback gain in said feedback control portion, and when saidovershooting is detected, a differential gain is increased as comparedwith the differential gain shown before said overshooting is detected.17. The electric brake system according to claim 14, wherein saidfeedback control portion executes the feedback control based on PID, andsaid feedback control variable correction portion corrects the feedbackgain in said feedback control portion, and when said overshooting isdetected, a differential gain is not changed and at least one of aproportion gain and an integral gain is decreased as compared with thegains shown before said overshooting is detected.
 18. The electric brakesystem according to claim 1, further comprising a determination portionfor determining that said overshooting has occurred if said overshootdetection portion has already detected said overshooting when saidtarget value and said actual control variable are in agreement in anincrease/decrease trend, wherein when said determination portiondetermines that said overshooting has been detected, said feedbackcontrol variable correction portion executes correction on said feedbackcontrol variable.
 19. The electric brake system according to claim 1,wherein said feedback control portion executes the feedback controlbased on PID, and said feedback control variable correction portioncorrects the feedback gain in said feedback control portion, and whensaid overshooting is detected, a differential gain is increased ascompared with the differential gain shown before said overshooting isdetected.
 20. The electric brake system according to claim 1, whereinsaid feedback control portion executes the feedback control based onPID, and said feedback control variable correction portion corrects thefeedback gain in said feedback control portion, and when saidovershooting is detected, a differential gain is not changed and atleast one of a proportion gain and an integral gain is decreased ascompared with the gains shown before said overshooting is detected.